Excavators used for digging slurry walls range in size from approximately 190,000 to 240,000 or more pounds. These Excavators are equipped with custom sticks and/or booms, to extend the digging depths from 50 feet to 90 feet or more.
When used to excavate slurry walls, the trenching excavator starts at the ground surface and digs to the required depths per engineer requirements. The operating cost of the machine is the same when digging at shallow depths and at the maximum depths. But typically, there is no way around using a larger, more expensive excavator (e.g., costing $50,000 to $80,000 or more per month) to dig both the shallow and deeper depths, even though a less expensive machine could handle the shallower depths. That is, the contractor often has to start the job with a machine capable of digging to the maximum required depths on the plans/design because it is not cost effective to have the larger excavator move out of the way to allow a smaller machine to dig first. In addition, the larger machine would just sit dormant while the smaller machine digs and vice versa. Therefore, this would simply increase the overall cost, for little if any increase in production efficiency (i.e., having just increased the cost for very little increase in production, if any).
However, larger excavators are difficult to operate with an open, continuous trench in slurry wall construction. Large excavators are cumbersome, and still risk collapsing the trench when straddling the trench trying to reset the mats on a new set. The larger machines cannot straddle the trench for very long to allow use of two machines working on the same heading. The excavators weigh too much and the distance between the tracks are too narrow (the trench might collapse).
Mats can be used, but the footprint is still small, and still risk collapsing the trench. In addition, moving mats take a long time, as sections of the mat need to be moved one at a time, while still having to support the excavator with other sections of the mats, which can be very time consuming. Thus, typically the only time contractors use mats are when they are working on unstable/soft soil.